Bash debugging tips

There are quite a few things that can be done with bash to help debugging your scripts containing the rulesets. One of the first problems with finding a bug is to know on which line the problem appears. This can be solved in two different ways, either using the bash -x flag, or by simply entering some echo statements to find the place where the problem happens. Ideally, you would, with the echo statement, add something like the following echo statement at regular intervals in the code:

...

echo "Debugging message 1."

...

echo "Debugging message 2."

...

In my case, I generally use pretty much worthless messages, as long as they have something in them that is unique so I can find the error message by a simple grep or search in the script file. Now, if the error message shows up after the "Debugging message 1." message, but before "Debugging message 2.", then we know that the erroneous line of code is somewhere in between the two debugging messages. As you can understand, bash has the not really bad, but at least peculiar, idea of continuing to execute commands even if there is an error in one of the commands before. In netfilter, this can cause some very interesting problems for you. The above idea of simply using echo statements to find the errors is extremely simple, but it is at the same time very nice since you can narrow the whole problem down to a single line of code and see what the problem is directly.

The second possibility to find the above problem is to use the -x variable to bash, as we spoke of before. This can of course be a minor problem, especially if your script is large, and if your console buffer isn't large enough. What the -x variable means is quite simple, it tells the script to just echo every single line of code in the script to the standard output of the shell (generally your console). What you do is to change your normal start line of the script from this:

#!/bin/bash

Into the line below:

#!/bin/bash -x

As you will see, this changes your output from perhaps a couple of lines, to copious amounts of data on the output. The code shows you every single command line that is executed, and with the values of all the variables et cetera, so that you don't have to try and figure out exactly what the code is doing. Simply put, each line that gets executed is output to your screen as well. One thing that may be nice to see, is that all of the lines that bash outputs are prefixed by a + sign. This makes it a little bit easier to discern error or warning messages from the actual script, rather than just one big mesh of output.

The -x option is also very interesting for debugging a couple of other rather common problems that you may run into with a little bit more complex rulesets. The first of them is to find out exactly what happens with what you thought was a simple loop, such as an for, if or while statement? For example, let's look at an example.

#!/bin/bash

iptables="/sbin/iptables"

$iptables -N output_int_iface

cat /etc/configs/machines | while read host; do

$iptables -N output-$host

$iptables -A output_int_iface -p tcp -d $host -j output-$host

cat /etc/configs/${host}/ports | while read row2; do

$iptables -A output-$host -p tcp --dport $row2 -d $host -j ACCEPT

done

done

This set of rules may look simple enough, but we continue to run into a problem with it. We get the following error messages that we know come from the above code by using the simple echo debugging method.

work3:~# ./test.sh

Bad argument `output-'

Try `iptables -h' or 'iptables --help' for more information.

cat: /etc/configs//ports: No such file or directory

So we turn on the -x option to bash and look at the output. The output is shown below, and as you can see there is something very weird going on in it. There are a couple of commands where the $host and $row2 variables are replaced by nothing. Looking closer, we see that it is only the last iteration of code that causes the trouble. Either we have done a programmatical error, or there is something strange with the data. In this case, it is a simple error with the data, which contains a single extra linebreak at the end of the file. This causes the loop to iterate one last time, which it shouldn't. Simply remove the trailing linebreak of the file, and the problem is solved. This may not be a very elegant solution, but for private work it should be enough. Otherwise, you could add code that looks to see that there is actually some data in the $host and $row2 variables.

The third and final problem you run into that can be partially solved with the help of the -x option is if you are executing the firewall script via SSH, and the console hangs in the middle of executing the script, and the console simply won't come back, nor are you able to connect via SSH again. In 99.9% of the cases, this means there is some kind of problem inside the script with a couple of the rules. By turning on the -x option, you will see exactly at which line the script locks dead, hopefully at least. There are a couple of circumstances where this is not true, unfortunately. For example, what if the script sets up a rule that blocks incoming traffic, but since the ssh/telnet server sends the echo first as outgoing traffic, netfilter will remember the connection, and hence allow the incoming traffic anyways if you have a rule above that handles connection states.

As you can see, it can become quite complex to debug your ruleset to its full extent in the end. However, it is not impossible at all. You may also have noticed, if you have worked remotely on your firewalls via SSH, for example, that the firewall may hang when you load bad rulesets. There is one more thing that can be done to save the day in these circumstances. Cron is an excellent way of saving your day. For example, say you are working on a firewall 50 kilometers away, you add some rules, delete some others, and then delete and insert the new updated ruleset. The firewall locks dead, and you can't reach it. The only way of fixing this is to go to the firewall's physical location and fix the problem from there, unless you have taken precautions that is!